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Spatially and time varying inflow - turbine simulation

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Old   July 16, 2012, 04:45
Default Spatially and time varying inflow - turbine simulation
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Alexander Schlüter
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Hello everybody,


I want to simulate the flow at a tidal turbine rotor. I am new to CFD but I have to simulate a inflow which should be allowed to vary in time and also in space.

My goal is to obtain dynamical loads and power characteristics of the turbine.


As a fist step I thought of including yaw misalignment (inclined inflow relative to turbine axis) by the use of mesh interfaces. The turbine is fixed in a sphere mesh that can be rotated relative to an outer mesh.


But I have honestly no idea about how to account for time or spatially varying inflow. Can UDFs be a possible solution or do you have any other hints for me about how to tackle the problem?


Thank you very much for your help!

Alex
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Old   July 16, 2012, 20:42
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Patrick
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Quote:
Originally Posted by Alex_S View Post
Hello everybody,

I want to simulate the flow at a tidal turbine rotor. I am new to CFD but I have to simulate a inflow which should be allowed to vary in time and also in space.

My goal is to obtain dynamical loads and power characteristics of the turbine.

As a fist step I thought of including yaw misalignment (inclined inflow relative to turbine axis) by the use of mesh interfaces. The turbine is fixed in a sphere mesh that can be rotated relative to an outer mesh.

But I have honestly no idea about how to account for time or spatially varying inflow. Can UDFs be a possible solution or do you have any other hints for me about how to tackle the problem?

Thank you very much for your help!

Alex
Hi Alex,

UDFs are a possible solution for time/spatially varying flows, but it's not advisable to use CFD for your application, in my opinion (at uni I've done CFD and tidal turbine design projects). I assume you want to see how the turbine behaves over a tidal cycle or something like that? Unless you are looking at quick changes in inflow condition (unlikely in the sea), there is no point in using CFD in my opinion.

If you do want to use CFD, it might be better to take several 'snapshots' of the flow with constant inlet conditions (although these can be spatially varying of course, this is easy to do with UDFs.) So yes it's possible, but CFD is overkill if this is a uni design or personal project.

Here are some other ways you can get the loads, power etc:

Blade element momentum theory for axial turbines.

Stream tube model for vertical ones, see:

Beri, H et al. (2011) “Double Multiple Stream Tube Model and Numerical Analysis of Vertical Axis Wind Turbine”, Energy and Power Engineering, 3(1), pp. 262-270, Scientific Research, [Online], Available at: http://www.SciRP.org/journal/epe (Accessed: 5 December 2011).
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Old   July 17, 2012, 03:38
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Alexander Schlüter
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Hi Patrick,

Thank you very much! I think you are right. In fact I have already implemented a BEM tool in MATLAB to compute loads and energy yield for a whole tidal cycle (or even a moon cylce).

I have planned to investigate inclined inflow (for this case I dont need to model spatially or time varying inflow) and also big waves or turbulence hitting the turbine. In the second case I definitely need time varying inflow conditions and I have intended to simulate the whole wave (about 15s).

Is that somehow realistic? Yesterday I have done some research on UDFs and as you said I think it should not be too difficult.

It would be also very interesting to simulate big waves which may result in backwards flow at some time step.

I have intended to define a velocity inlet boundary at the inflow and a pressure outlet at the outflow boundary. My first thought would be to simply define negative velocities at the inlet, but is this reasonable regarding the pressure outlet?

Thank you!
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Old   July 18, 2012, 07:26
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Originally Posted by Alex_S View Post
Hi Patrick,

Thank you very much! I think you are right. In fact I have already implemented a BEM tool in MATLAB to compute loads and energy yield for a whole tidal cycle (or even a moon cylce).

I have planned to investigate inclined inflow (for this case I dont need to model spatially or time varying inflow) and also big waves or turbulence hitting the turbine. In the second case I definitely need time varying inflow conditions and I have intended to simulate the whole wave (about 15s).

Is that somehow realistic? Yesterday I have done some research on UDFs and as you said I think it should not be too difficult.

It would be also very interesting to simulate big waves which may result in backwards flow at some time step.

I have intended to define a velocity inlet boundary at the inflow and a pressure outlet at the outflow boundary. My first thought would be to simply define negative velocities at the inlet, but is this reasonable regarding the pressure outlet?

Thank you!
The UDFs wont be too difficult in principle. The meshing and setting the rest of the problem up, will be hard, however, especially if you are new to CFD. I can send you an example UDF for a time/spatially varying flow in a 2d axisymmetric pipe if you want.

You also won't have much of an idea how accurate your simulations are unless you have some experimental data to compare against. You could at least compare to the BEM results to begin with. I'm also wondering how affected by turbulence/waves a tidal turbine is, given that they are usually submerged. The flow speeds up but the wave doesn't 'hit' the turbine underwater, so in this case I would just used the BEM with the higher flow speeds. The flow is always turbulent in tidal turbines /propellors that have been proven to work in real life so I'm also wondering how useful looking at the effects of turbulence might be in the first place. If there's a place where CFD could be more useful it would be to look at the effects of cavitation on the blades.

So I'm not trying to put you off but if this is a big task, potentially your time could be better spent elsewhere (I'm speaking from experience).

Yes you can use negative inlet velocities with the velocity inlet/pressure outlet, look in the fluent manual for some guidelines. I don't think the other BC types are suitable however.
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Old   July 19, 2012, 02:45
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Alexander Schlüter
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Hi Patrick,

Thank you very much!

The plan with CFD is to get an idea of the dynamic loads acting on the turbine if a wave ( which induces time varying and spatially varying velocities that are comparable to the mean flow near the surface) hits the rotor.
If I say hit I do not mean that the turbine is above the sea surface but that the wave induced velocities in some waterdepth hit the turbine. I do not plan to simulate an emergence of the turbine.

The other problem which I want to describe using CFD is inclined inflow. Again I want to investigate the loads acting on the turbine and also the power yield. Both will be dependend on the inflow angle.

Cavitation has already been investigated in steady calculations but I could do it again for inclined inflow and the "wave case".

Dont you think that CFD can be useful in both cases?
Actually I have intended to use CFD-results as some kind of validation tool for BEM calculations because I thought CFD would be more accurate ( when done right).

Of course I am very interested in your UDF (alexander.schlueter86@gmx.de). Thank you again!

Alex

Last edited by Alex_S; July 27, 2012 at 09:21.
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